Chemical plating method, electrolytic cell and automotive oxygen sensor using it

ABSTRACT

An automotive lambda oxygen sensor is formed by electroless plating of a thin, catalytically active, conductive electrode uniformly on the outer surface of a zirconia thimble. The process includes forming a pristine zirconia solid electrolyte thimble and drilling out a cylindrical cavity in it. A porous outer surface suitable for producing crystallization sites is formed by dipping the unfired thimble in a zirconia slurry containing spray-dried microspheres and firing the coated thimble to densify the thimble and the microspheres and to produce cavities on the surface of the thimble. An inner platinum electrode is formed by conventional conductive ink painting on the axial cavity of the sensor, and the sensor is again fired. The surface is activated by immersion in an acetone chloroplatinic acid bath to form multiple crystallization points, heat treated, then plated in an electroless platinum bath to a desired thickness. After plating, the sensor is heat treated and a conventional spinel glaze coat is flame sprayed over the sensor. The process produces sensors which consistently provide rapid response times and stable operation.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This is a division of copending application Ser. No. 08/953,864,filed Oct. 16, 1997, including continued prosecution application filedMay 10, 2000, now U.S. Pat. No. 6,315,880.

BACKGROUND OF THE INVENTION

[0002] This invention relates to solid state electrolytic cells and tooxygen sensors utilizing them. It has particular utility as a highlystable, rapid response lambda oxygen sensor in an automotive exhaustsystem.

[0003] Solid state electrolytic cells are well known. A particularlyuseful cell includes a solid electrolyte which selectively transmitsoxygen and which includes catalytic electrodes on opposed sides of thesolid electrolyte. Such cells are widely used as automotive lambda(stoichiometric) exhaust gas sensors, where they produce a voltagesignal which is highly dependent on the amount of oxygen in the exhaustgas stream. It will be understood, however, that the usefulness of theinvention is not limited to such sensors. For example, multiple suchcells can be connected as non-stoichiometric, pumping oxygen sensors.See, for example, Kondo et al., U.S. Pat. No. 5,480,535. In other uses,when connected as a current generator, such cells act as fuel cells, andwhen an external voltage is applied, they can act as oxygen generatorswhich produce exceptionally pure oxygen.

[0004] A common configuration of an automotive lambda exhaust gas sensoris a small thimble-shaped body of compacted zirconia (zirconium dioxide)stabilized with about 2-10 mole percent yttria (Y₂O₃) and, optionally,0-20 mole percent alumina (Al₂O₃). The catalytic electrodes can bepainted on as a platinum ink. Commonly, the outer electrode is formed byvacuum sputtering a thin film onto substantially the entire outersurface of the thimble. The sputtering process is expensive andinefficient, the electrodes are of varying thickness from one axial endof the thimble to the other, and the resulting sensors are unpredictableand have high reject rates.

[0005] The basic operation and known problems of an automotive lambdaexhaust gas sensor are described, for example, in Topp et al., U.S. Pat.No. 3,978,006, Burgett et al., U.S. Pat. No. 3,844,920, Romine et al.,U.S. Pat. No. 4,186,071, and Berg et al., U.S. Pat. No. 4,253,934. Asset out in these patents, it is desirable for the sensor to haveswitching times on the order of under 200 milliseconds when theair-to-fuel ratio fed to the engine switches from lean to rich or richto lean with respect to the stoichiometric ratio. It is also desirablefor the sensor to produce smooth switches of at least about 200 to 300millivolts when the air-fuel ratio switches. In recent years, the timerequired for an oxygen sensor to reach its operating temperature hasalso been recognized as a significant problem, and heated oxygen sensorshave become standard. It is thus also desirable to produce an oxygensensor which is well suited to introduction of a heater into the sensorstructure. The background of heated oxygen sensors is well set out, forexample, in Ker et al., U.S. Pat. No. 4,824,550.

SUMMARY OF THE INVENTION

[0006] One object of the present invention is to provide a solid-stateelectrolytic cell which is simple and inexpensive to manufacture.

[0007] Another object is to provide such a cell which, when utilized asa lambda oxygen sensor produces rapid response times and high signalstrength.

[0008] Another object is to provide such a cell which is reliable andreproducible.

[0009] Another object is to provide such a cell which is easilyadaptable to use with a heater.

[0010] Another object is to provide a simple, reliable, high-performanceoxygen sensor which incorporates such a cell.

[0011] These and other objects will become apparent to those skilled inthe art in light of the following disclosure and accompanying drawings.

[0012] In accordance with one aspect of the invention, generally stated,a solid electrolyte cell is provided comprising a solid electrolyte bodyhaving a first side and a second side, a first electrode on the firstside of the body, the first side of the body having a porous surfacecomprising a plurality of substantially spherical recesses, a firstelectrode substantially covering the first side of the body, the firstelectrode comprising a thin layer of conductive catalytic materialextending into the recesses to mechanically lock the layer to the firstsurface, and a second electrode on the second surface of the body. Thecell is preferably an oxygen sensor installed in the exhaust system of acombustion system, most preferably of an internal combustion engine. Ina preferred embodiment, the cell is a lambda oxygen sensor formed as athimble, the first surface being the outside of the thimble. The layeris plated on the first surface and is of substantially uniform thicknessfrom a closed axial end of the thimble to near an open axial end of thethimble.

[0013] Preferably the solid electrolyte is a yttria-stabilized zirconia,having an yttria content of about two to ten percent yttria, mostpreferably having a mole percentage of yttrium of about 3-6% and a molepercentage of alumina of zero to twenty percent. The electrodes arepreferably formed of platinum, rhodium, or palladium, most preferablyplatinum.

[0014] In accordance with another aspect of the invention, a method isprovided of forming a solid electrolyte cell, the method comprising astep of forming a solid electrolyte body including a porous layer on afirst surface of the body, a step of activating the first surface of theporous layer to form a plurality of growth points for a conductive layeron the first surface, a step of forming a first electrode by plating aconductive layer on the activated first surface of the body, and a stepof forming a second electrode on a second surface of the body.Preferably, the porous layer comprises substantially spherical recesseswhich are formed by coating the body with a slurry of solid electrolyteincluding in the slurry spray-dried balls of the electrolyte. On firingthe body, the spray-dried balls are densified to form small balls ofsolid electrolyte at the bottoms of substantially spherical recesses.The plating process preferably includes activating the first surface bydipping the porous layer of the body in a solution of platinum salt in avolatile solvent, such as acetone, and allowing the solution to wickinto the porous layer. The preferred platinum “salt” ishexachloroplatinic acid, and the term “salt” as used throughout thespecification and claims will be understood to include acids. Thesolvent preferably wets the ceramic. The body is then fired to drive offthe solvent and reduce the platinum salt to a 0.01 to 0.5 micron layerof platinum, with numerous unplated areas. The activated body is thenplated by electroless plating procedures to grow a coating of about oneto ten microns of platinum on the first surface. The coating ispermeable to oxygen at the intersections of crystals emanating fromindividual activation sites. The platinum coating is mechanically lockedinto the spherical recesses during the plating process.

[0015] In accordance with another aspect of the invention, a method isprovided of forming a solid electrolyte cell, the method comprising astep of forming a body including an elongate body formed of a solidelectrolyte compact, thereafter a step of drilling an axial cavity inthe body, and thereafter a step of firing the body to densify it.Preferably the body is formed by uniaxially compressing a zirconiapowder into a thimble having a tapered bore, and then drilling out thetapered bore to form a substantially cylindrical cavity.

[0016] In accordance with another aspect of the invention, an oxygensensor is provided including a thimble-shaped electrolytic cell havingan interior defined by a substantially cylindrical wall, an electricalcontact on the wall, an elongate electrical terminal extending fromoutside the cell into the interior of the cell, the terminal including apair of arms, at least one of the arms engaging the contact on the wall,and an elongate electrical heater extending into the interior of thecell, the terminal arms embracing the heater and positioning the heaterin the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a cross-sectional view of a preferred oxygen sensor ofthe present invention.

[0018]FIG. 2 is an exploded view of the sensor of FIG. 1.

[0019]FIG. 3 is a cross sectional view of a photomicrograph of a surfaceof an electrolytic cell of the oxygen sensor of FIGS. 1 and 2, plated inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0020] Referring now to the drawings, and in particular to FIGS. 1 and2, reference numeral 1 indicates a preferred embodiment of automotiveexhaust gas lambda oxygen sensor of this invention. The sensor 1 is inmost respects similar in construction to that shown in FIGS. 1-8 ofWolfe, et al., U.S. Pat. No. 5,049,255, and to the construction of anoxygen sensor commercially available from Tomco, Inc., of St. Louis, Mo.The overall construction and operation of the sensor 1 are well known tothose skilled in the art.

[0021] In brief, the oxygen sensor 1 includes a cell 3, a lower body 5,an upper body 7, a shroud 9, a wave washer 11, a spacer 12, a graphiteseal 13, an insulator 15, a terminal 17, a heater assembly 19, a button21, a grommet 23, a debris shield 25, and a tamper-proof shield 27.

[0022] The cell 3 includes a body 31 formed as a thimble, i.e., as ahollow conical frustum having a closed lower end 33 defined by a walland an open upper end 35. The body 31 is about 2.5 cm tall, 1.0 cm indiameter at its upper end and 0.6 cm in diameter at its lower end. Thebody flairs slightly at its upper end. A central axial bore 37 has adiameter of 0.3 cm and a depth of about 2.3 cm. The body 31 is formed of5% yttria stabilized zirconia. On the exterior of the body 31 is auniform coating 39 of platinum, the coating having a thickness of aboutone to twenty microns, preferably about one to ten microns. The coating39 extends to about 0.4 cm from the top edge of the body. The wall ofthe bore 37 includes a platinum stripe 41 extending from the bottom ofthe interior bore 37 to the top of the bore, the stripe having athickness of about ten to sixty microns.

[0023] The lower body 5 is internally tapped and threaded to form a borewhich receives the seal 13, cell 3, wave washer 11 and spacer 12, all ofwhich are held snugly in place by the externally threaded upper body 7.The graphite seal 13 is pressed against the lower body 5 and forms anelectrical connection between the outer face of the cell 3 and the body.The shroud 9 is welded to a lower end of the lower body part 5 andprotects the lower end of the cell 3.

[0024] The upper body 7 includes an axial bore, of smaller diameter thanthe bore of the lower body 5, which receives the insulator 15. Theinsulator 15 is a ceramic sleeve which electrically isolates theterminal 17 from the body 5 and 7.

[0025] The terminal 17 extends through the insulator 15, spacer 12, andwave washer 11, into the bore 37 of the cell 3. Two lower arms 43 andtwo upper arms 45 are provided at the lower end of the terminal 17. Theupper terminal arms 45 are proportioned to form a good electricalcontact with the platinum stripe 41 on the inside of the body 3 and tohold the terminal 17 frictionally in the cell 3. The lower arms 43 areproportioned to receive the heater assembly 19, rather than an electricwire as in Wolfe, et al., U.S. Pat. No. 5,049,255. Electrical connectionis made to the terminal 17 by welding or crimping a lead wire (notshown) to a head part 47.

[0026] The heater assembly includes an elongate sheath 49 having aresistance heater 51 at its lower end with leads running through thesheath to terminals 53 at the upper end of the heater. The terminals 53have welded or crimped to them electrical wires (not shown). The heater51 extends to near the bottom of the bore 37 of the cell 3. The lowerterminal arms 43 surround, support, and guide the sheath 49 of theheater assembly 19 to maintain its axial position in the bore 37 of thecell 3. Although the oxygen sensor 1 will operate without the heaterassembly 19, the cell will be brought to operating temperature far morequickly by operating the heater 51 when the automobile engine isstarted, as is well known in the art. The design of the heater assembly19 and uniform cylindrical bore 37, provide rapid and uniform heating ofthe cell 3, to provide rapid warm-up times for the cell, therebydecreasing pollutants more quickly when the engine is started.

[0027] The upper end of the upper body 7 is closed by the button 21 andthe grommet 23, which is held by the turned upper edge of the debrisshield 25. The debris shield is friction-fitted to the upper end of theupper body 7, and the tamper-proof shield 27 is friction fitted over itand a hex-nut portion of the upper body 7 to discourage disassembly ofthe body. The button and grommet have bores in them aligned with theterminals 53, to permit passage of the wires welded to the head 47 andterminals 53.

[0028] The constructions of the parts other than the cell 3 are wellknown to or easily determined by those skilled in the art.

[0029] The cell 3 is constructed as follows.

[0030] A 5% yttria zirconia powder in an acrylic binder is lubricatedwith a fatty acid such as palmitic acid in an ethanol vehicle. Thepowder has an average particle size of less than one micron. The powderis dried in air and uniaxially pressed at a pressure of 2,000 to 15,000pounds, preferably 3,000 to 4,000 pounds, into a thimble compact havinga mirror outer surface. A tapered mandrel forms a central tapered borein the compact. The compact is bored with a diamond drill to form auniform cylindrical bore having a central point at its lower endremaining from the bore formed by the mandrel. Drilling the bore, ratherthan machining the exterior of the thimble compact as is generally done,5 reduces the labor required. The compact is then dipped in an alcoholslurry of stabilized zirconia powder and spray-dried stabilized zirconiagranules to deposit a coating about fifty microns thick. A preferredcomposition of the slurry is: 4.5 mole percent yttria-stabilizedzirconia with acrylic binder 48.00 g. (spray dry granules - 200-250mesh) 3.0 mole percent yttria-stabilized zirconia 24.83 g. (powder - <1μ particles) Y₂O₃ (1-5 μ particles) 0.77 g. Al₂O₃ (1-10 μ particles)6.40 g. EtOH (denatured absolute) 187 ml. Fish oil 2.67 g. Polyvinylbutanol (PVB) 0.85 g.

[0031] The ethanol and fish oil are shaken until dissolved. Thestabilized zirconium oxide powder, yttria, and alumina are added androlled overnight. PVB is added and rolled thirty to forty-five minutes,then most of the beads are removed. The spray-dried granules are addedand rolled five minutes. The mixture is agitated to maintain thegranules in suspension.

[0032] The coated compact is dried in ambient air and then fired to atemperature of 1440° C. and held for two hours in air. Firing isaccomplished in stages; first raising the temperature to 350° C. overseven hours, holding for one hour, then raising to 550C over seven hoursand holding three hours, before raising to 1440° C. for two hours. Thepart is cooled rapidly, at a rate of 5° C. per minute. The firingprocess burns off the acrylic binder and reduces the dimensions of thethimble by about twenty-five percent. The resulting thimble has a bodywhich is smooth, dense and nonporous, covered with an external coating55 which is highly porous. The coating is chemically bonded to the body.If the body were formed entirely of the coating, it would be worthlessas a solid electrolyte for an oxygen sensor, because it would conductair. In the firing process, the spray-dried granules in the coatingshrink away from the matrix forming the coating and form spherical voids57 in the matrix, with the densified granules bonded to their interiors.These spherical voids play an important part in the plating process asdescribed hereinafter. The porous coating also includes many smallervoids which likewise play an important role in the plating process.

[0033] After the compact has been fired and densified to form the body,interior and exterior electrodes are applied.

[0034] The interior electrode 41 is painted on as a stripe of platinumink, to form a thick film electrode. The thimble body is again fired inair to a temperature of 1280° C. and held for two hours.

[0035] The thimble body is cooled, then dipped in an activation bathcontaining about fifty grams of platinum as hexachloroplatinic acid(122.3 g hexachloroplatinic acid hexahydrate) per liter of acetone. Thesolution is wicked up into the porous coating 55, and the platinumdeposits on discrete sites on the surface. The solution preferably doesnot wick onto the upper 0.3 cm of the thimble body. The activatedthimble is then dried and fired in air to 700° C. for two hours. Theactivation process produces a large number of nucleation sites having acoating of pure platinum with a thickness of about 0.01 to 0.5 microns,preferably 0.1 to 0.5 microns.

[0036] The activated thimble is immersed in boiling water for twominutes, then immersed in cold dilute hydrochloric acid (pH 2 to 5),then immersed in an electroless plating solution which is raised intemperature from room temperature to 80° C. and held for approximatelyforty minutes. The plating solution preferably has the followingcomposition: Distilled water 375 ml. Concentrated HCl (30%) 32 mlEthanol (denatured 200 proof) 2.75 ml Chloroplatinic acid (0.1 g./ml.Pt) 23.0 ml Hydrazine dihydrochloride (0.200 g./ml.) 11.5 ml

[0037] Dilute with distilled water to 458 ml.

[0038] The foregoing solution will plate eighty-eight thimblessimultaneously to a thickness of about three microns, while depletingthe plating bath. Coatings from about one to about fifteen microns arebelieved to produce acceptable sensors, although the acceptablethicknesses are determined empirically. In theory, any coating which isconductive (provides electrical continuity) and which permits oxygen topermeate the solid electrolyte body should be operable. Because nearlyall of the platinum in the plating solution is applied to the parts, andthe remainder is easily recovered, the process is extremely efficientand cost-effective.

[0039] The temperature of the plating solution is also determinedexperimentally for a particular purity and source of chloroplatinicacid, the temperature being chosen to provide complete plating withoutprecipitation of the platinum.

[0040] The electroless plating process provides coatings of greatuniformity. As shown in FIG. 3, unlike the results of painting a thickfilm ink onto the surface or sputtering a film onto the surface, theplated film extends into the pores of the porous coating, including thespherical openings produced by the densified granules in the coating.The platinum film is thus locked into the pores and cannot be peeledfrom the surface of the thimble. Because the film is grown from a largenumber of nucleation sites, numerous intersecting crystals are formed,which provide numerous domain boundaries. The film is of uniformthickness from the bottom of the cell to the top of the coating, unlikea sputtered coating which is much thicker at one end.

[0041] After the plating step is completed, the cell 3 is rinsedrepeatedly in distilled water and fired in air to 700° C. to bum off anyimpurities. The cell is then flame sprayed to give it a protectivespinel coating, as is conventional in the art.

[0042] The completed cell 3 is assembled into a sensor as shown in FIG.2. The sensor was tested in a 1988 Oldsmobile against other commerciallyavailable oxygen sensors and was found to have operating characteristicsbetter than all but the best. It has switching times of about 160milliseconds and prompt, accurate switches from 600 millivolts to 300millivolts in a snap throttle test. Even without the heater, it reachesoperating temperatures moderately quickly and operates well at lowertemperatures, such as idle temperatures. The thin wall and aspect ratio(length-to-diameter) of the cell 3 provide rapid heating of the cellboth by the heater 19 and by ambient exhaust gasses. It is believed thatstill better results may be obtained with different thicknesses of theexterior electrode 39 and by applying a more uniform inner electrode.

[0043] Numerous variations in the cell, method and sensor of the presentinvention, within the scope of the appended claims, will occur to thoseskilled in the art in light of the foregoing disclosure. For example,the body of the cell may include up to twenty percent alumina. Thealumina makes the cell physically stronger, draws silica impurities (sothat the grain boundaries are zirconia to zirconia), helps increasethermal conductivity, and reduces cost.

[0044] The cell, or a modification of it, can be used withnon-stoichiometric (e.g., pumping type) oxygen sensors of totallydifferent geometries.

[0045] It has been found that the sensor 1 is an efficient oxygengenerator when connected to a current source. Likewise, the cell may beused as a current generator when connected in an exhaust stream of acombustion process.

[0046] The plating technique may be used with other electrodes and toplate a precious metal on other substrates which have a porous surface.The porous surface can be a porous coating or, in accordance withbroader aspects of the invention, may be a part of the substrate itself.The precious metal may include gold, silver, the platinum metals(platinum, rhodium, palladium, osmium, ruthenium, and iridium), ormixtures thereof. The activation step may include forming nucleationsites of other metals, for example tin and palladium. These examples aremerely illustrative.

We claim:
 1. A solid electrolyte cell comprising a solid electrolytebody having a first side and a second side, a first electrode on thefirst side of the body, the first side of the body having a poroussurface of greater porosity than an underlying matrix of the body, theporous surface comprising a plurality of recesses, the first electrodesubstantially covering the first side of the body, the first electrodecomprising a thin layer of conductive catalytic material extending intothe recesses to mechanically lock the layer to the porous surface, and asecond electrode on the second side of the body.
 2. The cell of claim 1wherein the porous surface of the body comprises a plurality ofsubstantially spherical recesses and further comprises a small ball ofsolid electrolyte at the bottom of each of the substantially sphericalrecesses.
 3. The cell of claim 1 wherein the cell is a part of a lambdaoxygen sensor installed in the exhaust system of an internal combustionengine.
 4. The cell of claim 1 wherein the cell is a part of an oxygengenerator.
 5. The cell of claim 1 wherein the cell is formed as athimble, the porous surface being the outside of the thimble.
 6. Thecell of claim 5 wherein the layer is plated on the porous surface at asubstantially uniform thickness from a closed axial end of the thimbleto near an open axial end of the thimble.
 7. The cell of claim 1 whereinthe solid electrolyte is a yttria-stabilized zirconia.
 8. The cell ofclaim 1 wherein the first and second electrodes are formed of a materialselected from the group consisting of platinum, rhodium and palladium.9. The cell of claim 8 wherein the first and second electrodes areformed of platinum.
 10. A method of forming a solid electrolyte cellcomprising forming a solid electrolyte body, forming a porous layer on afirst surface of the body, activating the porous layer on the firstsurface of the body to form a plurality of growth points for aconductive layer on the first surface, growing a first electrode byelectroless plating of a conductive layer on the activated porous layeron the first surface of the body, and forming a second electrode on asecond surface of the body.
 11. The method of claim 10 wherein the stepof forming a solid electrolyte body comprises forming a body which isimpervious to air.
 12. The method of claim 10 wherein activating theporous layer on the first surface comprises wicking a metal salt carriedby a liquid into the porous layer.
 13. The method of claim 10 whereinthe body is formed as a thimble with an outer surface and an innersurface, the first electrode being formed on the outer surface.
 14. Themethod of claim 10 wherein growing a first electrode comprises immersionof the porous layer on the first surface in an unstable solution of asalt of a metal.
 15. The method of claim 14 wherein the unstablesolution further comprises a reducing agent
 16. The method of claim 15wherein the reducing agent comprises hydrazine.
 17. A method of forminga coating of a precious metal on a ceramic substrate, the methodcomprising a step of forming a ceramic substrate having pores at asurface of the substrate; a step of forming a solution of a salt of afirst metal in an organic solvent which wets the ceramic; a step offorming nucleation sites on the surface of the substrate, said step offorming nucleation sites including wicking the solution into the poresat the surface of the substrate; and thereafter an electroless platingstep of plating the precious metal onto the surface from an aqueousplating bath.
 18. The method of claim 15 wherein the organic solvent isacetone.
 19. The method of claim 15 wherein the first metal and theprecious metal are the same.
 20. The method of claim 15 including astep, after wicking the solution into the pores at the surface of thesubstrate, of heating the substrate to drive off the solvent and reducethe salt to a 0.01 to 0.5 micron layer of the first metal with numerousunplated areas.